FBG monitoring of curing in single fiber polymer composites

Abstract In this study, a fiber Bragg grating (FBG) sensor and a thermocouple (TC) were embedded inside an epoxy system cured at 121 °C to determine its curing characteristics. The curing behavior of this higher temperature (HT) system was also compared with a room temperature (RT) epoxy cured at 30 °C to independently verify the recently published approach developed for the RT system (Ref. 1; Hoffman, Khadka and Kumosa, 2020). The previously suggested algorithms developed to extract the gel point, the end of cure, and the curing and manufacturing strains for the RT system were successfully applied to the HT epoxy. This methodology was effectively extended to include the start of cure determination for the HT system. It was established that the manufacturing strains of both resin systems significantly exceeded the strains which could be predicted from the thermal contraction of the resins cooling from the ambient curing temperature to RT especially for the HT epoxy. The curing strains in both epoxy systems were shown to be significant, contributing to the overall manufacturing strains approximately 28% for the RT epoxy and about 45% for the HT resin. Efforts were also made to explain the source of sizable scatters in the strain measurements by evaluating temperature variations within individual and across multiple curing samples. As previously conjectured, the local temperature concentrations (hot spots) were responsible for the scatters in the strain measurements.